Diamond compacts and diamond-like-carbon (DLC) coatings have become an area of intense and growing development in both the scientific and industrial fields for many diverse applications, including aerospace, automotive, electronic, biomedical, and tooling. Diamond-like carbon designates dense, amorphous hydrocarbon structures embodying properties that differ markedly from those of other hydrocarbon structures, but instead have properties which in many respects resemble those of diamond. DLC coatings have a polycrystalline nature similar to that of diamond compacts which provide contact surfaces that exhibit low coefficients of friction (in contrast to monocrystalline coatings which typically exhibit a high coefficient of friction). In addition, diamond compacts and DLC coatings exhibit other diamond-like characteristics such as high wear resistance, high hardness, good corrosion resistance, chemical inertness, etc.
One major difference between diamond compacts and DLC coatings lies in the technique used to apply the compact or coating to a substrate. Diamond bodies are typically formed as micro-crystalline structures, i.e., extremely small crystals. One method of applying diamond compacts is achieved by bonding diamond grits of varying diameters, 20 microns to 1 millimeter, to the contact surface of a finished article. Bonding may be achieved by a variety of techniques. Epoxy and nickel plating techniques may be used to glue the diamond grits to the surface at temperatures in the range of 66.degree.-260.degree. C. (150.degree.-550.degree. F.) to form a surface grit layer having a thickness equivalent to the diameter of the diamond grits. Brazing and vitreous sintering techniques may also be used, with diamond grits being mixed in a matrix of powdered brazing metals or powdered glass. Braze-bonding requires temperatures in the range of 870.degree.-980.degree. C. (1600.degree.-1800.degree. F.) while vitreous bonding requires temperatures approaching 1090.degree. C. (2000.degree. F.). In general, diamond compacts provided by the foregoing techniques exhibit poor adhesion characteristics, poor fracture toughness, and are susceptible to the development and propagation of cracks.
Diamond coatings may be applied to a contact surface by means of a chemical vapor deposition (CVD) process. One method for applying diamond coatings to a contact surface is a plasma-assisted CVD process. Hydrogen-methane gas mixtures are excited, either by DC glow discharge or microwave activation, to produce a plasma in the vicinity of contact surface. Charged carbon particles are generated by the thermal decomposition of the methane gas component of the plasma, which is at a sufficiently high temperature to facilitate tetragonal carbon-carbon bonding, resulting in the condensation of a diamond film on the contact surface. The contact surface must be heated in excess of 800.degree. C. (1472.degree. F.) to promote film growth via deposition rates of about 1 micro per hour. CVD processes are disadvantageous in several respects. For example, the temperatures to which the contact surface is heated may deleteriously affect the contact surface, e.g., thermal growth that distorts the contact surface and loss of temper. In addition, the presence of plasma adjacent the contact surface may cause contamination of the contact surface, leading to loss of adhesion between the diamond coating and the contact surface.
Physical vapor deposition (PVD) processes may be used to deposit DLC coatings on contact surfaces. PVD processes such as magnetron sputtering and cathodic arc deposition form a plasma in the region adjacent to the contact surface where it ionizes an amorphous material disposed on the contact surface to form the DLC coating. Like CVD processes, the presence of plasma adjacent to the contact surface may cause contamination of the contact surface, which may lead to loss of adhesion between the DLC coating and the contact surface. Hybrid thermionically assisted PVD processes wherein solid graphite is evaporated by a differentially pumped, bent electron gun and directed into an RF or DC glow discharge or saddle field fast atom beam (FAB) PVD processes wherein a hydrocarbon gas is introduced into a cold-cathode source that generates ionizing electrons which experience oscillatory trajectories under the influence of a DC field may also be utilized to deposit DLC coatings on contact surfaces. While PVD processes such as the saddle field FAB technique possess certain advantages, e.g., contact surface to be coated is shielded from the plasma, the generated beam is uncharged, the saddle field FAB technique tends to produce DLC coatings having microstructure surface irregularities (see disclosure hereinbelow) that makes such DLC coatings unsuitable for applications wherein the DLC coating frictionally interacts with a non-DLC coated countersurface.
A need exists to provide an ion deposition coating method for depositing a DLC coating on the dynamic surface of an article wherein the DLC coating exhibits enhanced adhesion characteristics, high lubricity (low coefficient of friction), high hardness, high wear resistance, and good corrosion resistance. The ion deposition coating method should facilitate the deposition of a DLC coating on the dynamic surface of articles that are subject to poor adherence as well as to those articles which offer inherently good bonding to deposited DLC coatings.